1. Field of the Invention
This invention relates to surface features of implantable medical devices, for example stents and grafts, and to methods for forming such surface features.
2. Description of the Background
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress against and remodel the artery wall for dilating the lumen. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient""s vasculature.
A problem associated with the procedure includes formation of intimal flaps or torn arterial linings that can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an implantable device, an example of which includes an expandable stent, is implanted in the lumen to maintain the vascular patency. Stents are scaffoldings, usually cylindrical or tubular in shape, functioning to physically hold open, and if desired, to expand the wall of the passageway. Typically stents are compressed for insertion through small cavities via small catheters, and then expanded to a larger diameter once at the desired location. Examples in patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
A problem encountered with intravascular stents is that, once implanted into the blood stream, platelets and other blood components tend to adhere to any portion of the stent surfaces having roughness or irregularity. Adhesion and aggregation of platelets and other blood components can lead to thrombosis and restenosis. Therefore, an important aspect of manufacturing and finishing stents is ensuring that all stent surfaces are made extremely smooth, without roughness and irregularities. This is accomplished by highly polishing the entire surface of the stent material, typically by electropolishing or by using an abrasive slurry, as described in U.S. Pat. No 5,746,691 titled xe2x80x9cMethod for Polishing Surgical Stentsxe2x80x9d issued to Frantzen and U.S. Pat. No. 5,788,558 xe2x80x9cApparatus and method for Polishing Lumenal Prosthesesxe2x80x9d issued to Klein.
To further fight against thrombosis and restenosis, and in treating the damaged vascular tissue, therapeutic substances can be administered. For example, anticoagulants, antiplatelets and cytostatic agents are commonly used to prevent thrombosis of the coronary lumen, to inhibit development of restenosis, and to reduce post-angioplasty proliferation of the vascular tissue, respectively. To provide an efficacious concentration to the treated site, systemic administration of these drugs often produces adverse or toxic side effects for the patient. Local delivery is a highly suitable method of treatment in that smaller levels of medication, as compared to systemic dosages, are concentrated at a specific site. Local delivery therefore produces fewer side effects and achieves more effective results.
One commonly applied technique for the local delivery of therapeutic substances is through the use of medicated stents. A well-known method for medicating stents involves the use of a polymeric carrier coated onto the body of the stent, as disclosed in U.S. Pat. No. 5,464,650 issued to Berg et al., U.S. Pat. No. 5,605,696 issued to Eury et al., U.S. Pat. No. 5,865,814 issued to Tuch, and U.S. Pat. No. 5,700,286 issued to Tartaglia et al. The therapeutic substances are impregnated in, located on, or provided underneath the polymeric coating for release in situ once the stent has been implanted.
An obstacle often encountered with the use of stent coatings is poor adhesion of the polymeric coating to the surface of a stent. During stent delivery, a poorly adhering coating can be rubbed and peeled off of the stent if the coating contacts an arterial wall while the stent is being moved into position. Also, when a coated stent is expanded in situ, the distortion the stent undergoes as it expands can cause the coating to peel, crack, or tear, and disengage from the stent. Poor adhesion of the coating material can promote thrombosis and restenosis, by providing additional surfaces for platelets and other blood components to adhere. Additionally, poor adhesion and loss of the coating also leads to loss of a significant amount of the drugs to be delivered from the coating.
Another technical challenge in using stent coatings to deliver drugs is loading enough drug onto the stent, so that an effective amount of the drug or drug combination is delivered to the treatment site. The total amount of a drug that can be loaded onto a stent in a polymeric coating is limited by the amount of drug that can be mixed into the polymer (the concentration of the drug in the polymer), and the amount of polymer and drug mixture that can be coated onto the stent (the thickness of the coating on the stent for a given stent size). Therefore, a stent that carries more coating can deliver greater amounts of drugs. However, increasing the thickness of a stent coating can be difficult, particularly if the coating does not adhere well to the stent material.
When delivering drugs from a stent, it is also desirable to control the timing and rate of release of the drugs being delivered. Controlled release can be achieved by coating a stent with a number of layers. For instance, each layer can contain a different drug or be made of a polymer that releases drugs at different rates. However, additional layers tend to adhere poorly to underlying layers. The additional layers peel off either during application of the additional layer over an underlying layer, or, as described above, when the stent is delivered and expanded within the artery.
Another complication for drug delivery from a stent is that the arterial wall tissue the stent is compressed against can be tough and fibrotic, preventing medication released from the stent from penetrating the tissue in which the medication may be therapeutically beneficial.
An implantable medical device capable of delivering therapeutic substances from a coating is provided, which provides a high retention of one or more layers of coating material. The implantable device also allows a greater total amount of coating to be carried by the device, allowing for greater amounts of therapeutic substances to be delivered from the device. In some embodiments the implantable device can penetrate the arterial wall to enhance delivery of therapeutic substances into the arterial wall.
In one embodiment within the present invention, the implantable medical device has a generally tubular structure with an inner surface and an outer surface. The outer surface has asperities on designated regions that have roughness factors, Ra, of greater than 40 nm. The designated regions can be the entire outer surface, a middle section of the outer surface, or ends of the outer surface. Typically, the inner surface is smooth.
In various embodiments, the outer surface, or portion thereof, can be coated with a coating containing a therapeutic substance or substances, a polymer, or a combination of therapeutic substances and polymer. The coating can be made of one or more layers and the layers can hold different therapeutic substances, polymers, or combination of therapeutic substances and polymers.
The asperities may have surface protrusions and indentations of various shapes. In some embodiments, the asperities can have sharp tips, be rounded, be square, or a combination of these shapes.
Exemplary embodiments are made by polishing the inner surface of the implantable medical device and forming asperities on the designated region of the outer surface. In some embodiments, the inner surface is polished before the asperities are formed and the inner surface is protected while the asperities are being formed.
In some embodiments, the asperities are formed by projecting grit, which can be beads or sand, at the designated region of the outer surface.
In some embodiments, the asperities are formed by depositing material onto the designated region. In one method, the material is deposited by adding particles to the designated region and bonding the particles, for instance by sintering, to the designated region. In one method, the material is deposited by sputtering. In some embodiments, the material added to the outer surface is radio-opaque.
In some embodiments, the asperities are formed by applying a chemical etchant to the designated region and rinsing the chemical etchant off of the region after a predetermined period of time. The chemical etchant can by applied by sponging or spraying the etchant onto the outer surface. In some embodiments, a patterned mask, which has openings, is applied to the outer surface and then the chemical etchant is applied, allowing the chemical etchant to etch through the openings.
In some embodiments, the asperities are formed by machining or laser cutting.
These and other features and aspects of the various embodiments of the present invention may be better understood in view of the drawings and the following detailed description.